1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus and a process cartridge for electrophotographic image forming apparatus. In particular, the present invention relates to an electrophotographic image forming apparatus using a proximity charger, and a process cartridge therefor. In addition, the present invention also relates to a photoreceptor for use in the electrophotographic image forming apparatus, and to a method for manufacturing the photoreceptor.
2. Discussion of the Background
Recently the growth of electrophotographic information processing apparatus using a photoreceptor, such as copiers, printers and facsimiles, is remarkable. In particular, photo-printers capable of recording digital information using light have been drastically improving in recording qualities and reliability. This digital recording technique is applied to copiers as well as photo-printers. The copiers to which both the conventional analogue copying technique and this digital technique are applied have various image forming functions. Therefore it is considered that the demand for such copiers increases more and more.
In attempting to reduce the quantity of ozone and NOx generated in an electrophotographic image forming apparatus and the electric power consumption of the image forming apparatus when performing charging, charging methods using a charging roller have been proposed.
For example, Japanese Laid-Open Patent Publication No. (hereinafter referred to as JOP) 4-336556 discloses a contact charging device in which a charging roller charges a photoreceptor while contacting the photoreceptor. In this contact charging device, the surface of the charging roller is made of a dielectric material, and the rotating direction of the charging roller is the same as that of the photoreceptor used (i.e., at the contact point between the charging roller and the photoreceptor, the moving direction of the charging roller is opposite to that of the photoreceptor).
In this case, even when there is a pinhole on the surface of the photoreceptor, a problem in that a non-charged area is not formed on the area around the pin hole does not occur. This is because the surface of the charging roller is dielectric and therefore the charges of an area around the portion of the charging roller, which portion faces the pinhole of the photoreceptor, do not decay when performing charging. In addition, even when the photoreceptor and dielectric charging roller are frictionally charged due to friction between the photoreceptor and the charging roller, which rotates in a direction opposite to the rotating direction of the photoreceptor at the contact point thereof, a surface area of the photoreceptor to be charged can be contacted with a surface area of the charging roller having a relatively low charge potential (i.e., a surface area of the charging roller which is not the surface area having a high potential because of having been just rubbed with the photoreceptor). Thereby, the photoreceptor can be charged to a desired potential even when a relatively low voltage is applied thereto. Since the charging roller charges the photoreceptor while contacting the photoreceptor, the applied voltage is relatively low compared to non-contact chargers such as scorotrons, and therefore the quantity of the above-mentioned reactive gasses to be generated, such as ozone and NOx, can be reduced.
However, the contact charging devices have the following drawbacks:
(1) unevenly charging a photoreceptor (i.e., traces of the charging roller used can be observed in the resultant images) due to uneven contact of the charging roller with the photoreceptor used, etc.;
(2) producing large charging noise;
(3) charging ability deteriorates when toner particles, etc. present on the surface of the photoreceptor adhere on the surface of the charging roller;
(4) photosensitive properties of the photoreceptor change when one or more constituents of the charging roller adhere (migrate) to the photoreceptor; and
(5) the charging roller deforms when the photoreceptor is stopped for a long period of time, resulting in uneven charging.
The uneven charging mentioned above in item (1) is caused by adhesion of the constituents of the charging roller on the photoreceptor when the photoreceptor is stopped because the constituents migrate from the charging roller to the photoreceptor. The large noise mentioned above in item (2) is caused by vibrational contact of the charging roller with the photoreceptor. The vibration of a charging roller is caused when an AC voltage is applied to the charging roller.
In attempting to solve these problems, proximity charging devices have been proposed. In the proximity charging devices, a photoreceptor is charged by applying a voltage to a charger, which faces the photoreceptor while a narrow gap of from 0.005 mm to 0.3 mm is formed between the charger and the photoreceptor.
The proximity charging devices do not cause the problems mentioned above in items (4) and (5) because the charger does not contact the photoreceptor. In addition, with respect to the problem mentioned above in item (3), the proximity charging devices are superior to the contact charging devices because the quantity of toner particles adhered on the charger is less than in the case of the contact charging devices.
Proximity charging has been disclosed in, for example, JOPs 2-148059, 5-127496, 5-273837, 5-307279, 6-308807, 8-202126, 9-171282 and 10-288881.
These publications relate to proximity charging methods and it is described therein that a photoreceptor is experimentally charged with a charger while a gap is formed therebetween to observe whether the photoreceptor is evenly charged. However, there is no specific description in the publications as to how the charger is set closely to the photoreceptor, namely, ideas of constitution of proximity chargers are merely described therein. Actually, it is not easy to form a uniform gap not greater than hundreds of micrometers between a charger and a photoreceptor and stably maintain the gap. Namely, the proximity charging methods have a big problem of how to stably maintain a gap not greater than hundreds of micrometers between a charger and a photoreceptor.
In contrast, specific embodiments of a charger set closely to a photoreceptor are described in JOPs 5-107871, 5-273873, 7-168417 and 11-95523.
JOPs 5-107871 and 5-273873 have proposed a method in which an insulating tape whose ends are fixed by springs or the like and which serves as a gap forming member is set between a charger and a photoreceptor, to form a gap between the charger and the photoreceptor. This method is effective in forming a gap between a photoreceptor and a charger. However, when such a gap forming member is practically set in an image forming apparatus, a tension is applied to the springs in only one direction because the photoreceptor rotates in only one direction. Therefore, the springs are easily fatigued. In addition, when such a member is set in the image forming apparatus, the configuration of the resultant image forming apparatus becomes complex although this member has a simple mechanism. Therefore the maintenance of the image forming apparatus cannot be easily performed. For example, the image forming apparatus has a drawback in that when the gap forming member is changed, the photoreceptor has to be also changed.
JOP 7-168417 discloses a method in which a gap is formed between a photoreceptor and a charging roller by setting spacers on bearings of the charging roller, wherein the spacers contact the surface of the photoreceptor. In this case, the spacers have to be a part which is different from the charging portion of the charging roller in size and material, resulting in complication of the constitution of the charging roller. In addition, in this case the charging roller is made of an insulating material, and therefore a voltage applying roller which applies a voltage to the charging roller is needed, resulting in further complication of the constitution of the charging roller and increase of manufacturing costs of the charger.
JOP 11-95523 discloses a method in which a gap is formed between a charger and a photoreceptor by setting a gap forming member on at least one of the charger and the photoreceptor. This apparatus has a simple constitution, but there is no specific description about the specific constitution of the gap forming member and how to set the gap forming member. Therefore, the gap can be stably maintained (i.e., the photoreceptor can be stably charged) when the charging device is used for a long period of time.
JOP 4-360167 discloses a proximity charging device using a charger, on both ends of which a projected portion is formed to form a gap between the charger and a photoreceptor. By charging the photoreceptor with this charger while contacting the projected portion of the charger with the photoreceptor, proximity charging can be performed. However, there is no description about how to support the gap forming member and the photoreceptor and how to arrange the gap forming member relative to the image forming portion of the photoreceptor. Therefore, it is unknown whether a gap can be stably maintained (i.e., the photoreceptor can be stably charged) when the charging device is used for a long period of time.
In addition, there is no description about the measures against uneven charging around the edge portions of the photoreceptor close to the projected portions. Further there is no description about the measures against accumulation of toner particles on the edge portions of the photoreceptor close to the projected portions when the charger is repeatedly used. Therefore, it is unknown whether this proximity charging device can be stably used for a long period of time. Namely, the reliability of this proximity charging device is unknown in particularly when the charging device is practically used repeatedly.
JOP 7-121002 discloses an image forming apparatus in which a ring-form spacer is set on both ends of a cylindrical photoreceptor to form a gap between the photoreceptor and a charger. Around the photoreceptor, other devices such as an image developer, an image transferer and a cleaner are set while contacting the photoreceptor or being close to the photoreceptor. When such a ring spacer as mentioned above is set on both ends of the photoreceptor, the devices mentioned above cannot be provided on the ring spacer. Therefore the length of the photoreceptor in the axial direction needs to be extended to secure the desired image forming portion on the photoreceptor.
In addition, in this charging method charging near the ring spacers tends to become uneven (i.e., the potential on the edge portions tends to decrease). When such a charging method is used in combination with a nega-posi developing method which is suitable for digital image writing methods because image writing time can be saved, a problem such that background development is observed in these edge portions of the photoreceptor tends to occur.
Further, the spacers themselves and/or the charger tend to be contaminated. Therefore, the edge portions of the photoreceptor near the spacers should be cleaned such that there are no residual toner particles. However, since the spacers are formed on the photoreceptor, the edge portions cannot be cleaned. Accordingly, it is considered that this charging device has poor reliability when practically used repeatedly.
Because of these reasons, a need exists for a proximity charging device which has a simple constitution and in which a uniform gap is formed between the charger and a photoreceptor even when the charging device is repeatedly used.
Accordingly, an object of the present invention is to provide an electrophotographic image forming apparatus including a simple and low-cost proximity charging device which hardly causes the above-mentioned problems of the contact charging methods and which can be practically used. Specifically, a gap can be stably maintained between a charger and a photoreceptor without forming a toner film on the surface of the charger even when the charging device is repeatedly used.
Another object of the present invention is to provide an electrophotographic image forming apparatus including a proximity charging device which does not cause uneven charging, such as a banding problem specific to proximity charging, even in long repeated use, resulting in formation of good images for a long period of time.
Yet another object of the present invention is to provide a highly durable electrophotographic image forming apparatus and process cartridge, by which images having good image qualities can be stably produced even when repeatedly used without frequently changing the photoreceptor and charger due to abrasion resistance.
A further object of the present invention is to provide a photoreceptor for use in the electrophotographic image forming apparatus and process cartridge mentioned above.
A still further object of the present invention is to provide a method of manufacturing the photoreceptor.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophotographic image forming apparatus including at least a photoreceptor which rotates in a direction and which includes a gap forming member on both ends, wherein the photoreceptor includes an image forming portion having two ends substantially parallel to the rotating direction; a charger which is configured to charge the photoreceptor while rotating, wherein a gap is formed between the surface of the image forming portion of the photoreceptor and the periphery surface of the charger by the gap forming member and wherein the gap forming members do not contact the image forming portion of the photoreceptor; a light irradiator configured to irradiate the photoreceptor with light to form an electrostatic latent image in the image forming portion of the photoreceptor; an image developer configured to develop the latent image with a toner to form a toner image thereon; and an image transfer device configured to transfer the toner image onto a receiving material, wherein the following relationship is satisfied:
txe2x89xa72g 
where g represents the gap and t represents a distance between the inside edge of one of the gap forming members and one of the two ends of the image forming portion of the photoreceptor, which is closer to the inside edge of the one of the gap forming members.
The gap is preferably from 10 xcexcm to 200 xcexcm.
The gap forming members can be formed, for example, by forming a layer (projection) at the edge portions of the photoreceptor; by forming a thicker photosensitive layer at the edge portions of the photoreceptor than the photosensitive layer at the image forming portion thereof; by using a substrate for the photoreceptor, wherein the substrate has a thickness larger than that at the image forming portion thereof; or by providing a flange on the edge portions (i.e., non-image forming portions) of the photoreceptor.
The photoreceptor may be a belt-form photoreceptor which is supported and driven by at least a driving (or driven) roller. In this case, the width of the roller is longer than that of the belt photoreceptor, and the extended portions of the roller has a diameter larger than that of the central portion of the roller to form a gap.
It is preferable that at least one of the charger and the photoreceptor (or the driving or driven roller) is pressed toward the other by a spring, etc.
It is preferable that the rotating shaft of the charging roller is coupled with the rotating shaft of the photoreceptor by a ring member.
In addition, it is preferable that each of the charging roller and the photoreceptor has a respective driving device such as gears, couplings and belts so as to be independently rotated.
In another aspect of the present invention, a process cartridge is provided which includes at least the above-mentioned photoreceptor having a gap forming member on both ends; and the charging roller mentioned above, wherein a gap is formed between the image forming portion of the photoreceptor and the periphery surface of the charger, and wherein the following relationship is satisfied:
txe2x89xa62g 
wherein g represents the gap and t represents a distance between the inside edge of one of the gap forming members and one of the two ends of the image forming portion of the photoreceptor, which is closer to the inside edge of the one of the gap forming members.
In yet another aspect of the present invention, a photoreceptor is provided which includes at least an electroconductive substrate and a coating (i.e., a layer or layer) including at least a photosensitive layer located overlying the electroconductive substrate, wherein the thickness of the coating at the edge portions (i.e., the non-image portions) of the photoreceptor is thicker than that at the image forming portion of the photoreceptor. Alternatively, the thickness of the substrate at the edge portions may be greater than that at the image forming portion of the photoreceptor.
The photoreceptor may be provided with a flange on both ends thereof such that the flange covers the non-image portions, wherein the diameter of the flange is greater than that of the photoreceptor at the image forming portion.
It is preferable that the thickness difference of the coating or the substrate or the difference in diameter between the flange and photoreceptor at the image forming portion is preferably from 10 to 200 xcexcm.
The photosensitive layer preferably includes a charge generation layer and a charge transport layer located on the charge generation layer. The charge transport layer preferably includes a polycarbonate resin having a triarylamine unit in the main chain or side chain thereof.
The coating of the photoreceptor preferably includes a protective layer located overlying the photosensitive layer. The protective layer preferably includes a filler and/or a charge transport material.
In a further aspect of the present invention, a method for preparing the photoreceptor is provided which includes the steps of forming a coating including at least a photosensitive layer on a surface of an electroconductive substrate; and cutting a central portion of the coating, to form a thickness difference between the central portion and the edge portions thereof.
Alternatively, the method may include the steps of providing an electroconductive substrate in which the thickness (or diameter) of the edge portions is larger than that at the central portion, for example, by cutting; and forming a coating including at least a photosensitive layer on the surface of the edge portions and central portion of the surface of the electroconductive substrate.
The coating is preferably formed by a spray coating method.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.